In recent years, organic EL displays using electroluminescence (EL) of an organic material have been attracting attention as a type of next-generation flat panel display replacing liquid crystal displays. In an active-matrix display device such as the organic EL display, a thin-film semiconductor device referred to as a thin-film transistor (TFT) is used.
Unlike the voltage-driven liquid crystal display, the organic EL display is a current-driven display device. Accordingly, there is an urgent need for development of a thin-film transistor having excellent turn-on/off characteristics as a driving circuit for the active-matrix display device. The thin-film transistor includes a gate electrode, a semiconductor layer (channel layer), a source electrode, and a drain electrode formed above a substrate. Generally, a silicon thin-film is used as the channel layer.
Increased size and reduced cost are also required for the display device, and a bottom-gate thin-film transistor in which a gate electrode is formed on a side close to the substrate with respect to the channel layer is generally used as a thin-film transistor which allows cost reduction easily.
The bottom-gate thin-film transistor is generally divided into two types; namely, a channel etching thin-film transistor which includes an etched channel layer, and a channel protective (etching-stopper) thin-film transistor in which the channel layer is protected from etching.
Compared to the channel protective thin-film transistor, the channel etching thin-film transistor has advantages in the reduced number of photolithography process and a lower cost.
In contrast, with the channel protective thin-film transistor, it is possible to prevent the damage on the channel layer by etching, suppressing an increase in the variation of characteristics within a surface of the substrate. Furthermore, the channel layer of the channel protective thin-film transistor can be thinner than the channel layer of the channel etching thin-film transistor. Accordingly, parasitic resistance component can be reduced and the turn-on characteristics can be improved, which are advantageous for increasing definition.
Therefore, the channel protective thin-film transistor is suitable for a driving transistor in the current-driven organic EL display device using an organic EL element, for example, and there are attempts to use the channel protective thin-film transistor as a pixel circuit in the organic EL display device even if the manufacturing cost is higher than the manufacturing cost when the channel-etching thin-film transistor is used.
For example, the patent literature 1 discloses a channel protective TFT in which a microcrystalline semiconductor film is used as a channel layer, and a channel protective layer is formed above a channel layer with a buffer layer provided in between.